1. Field of the Invention
The invention relates to a gear for a vehicle steering system, having a pinion disposed on a shaft in a manner fixed against relative rotation, having a gear wheel that meshes with the pinion, the pinion and the gear wheel being prestressed in the radial direction.
2. Description of the Prior Art
Conventional vehicle steering systems, vehicle steering systems with an overriding gear, and steer-by-wire steering systems require one or more steering gears with which the rotary motion of the steering wheel is converted into a rotary motion of the steered wheels.
In conventional electric power steering systems, a torque brought to bear by an electric motor must be input into the steering system as well. In a steer-by-wire steering system, there is no mechanical or hydraulic connection between the steering wheel and the steered vehicle wheels. A steering actuator regulates the position of the steered vehicle wheels as a function of the driver's steering demand and such other variables as the yaw rate or the vehicle speed. The steering motion of the steered wheels can be programmed freely, and all the steering work is brought to bear by the electrical or hydraulic steering actuator.
In vehicle steering systems with an overriding gear, a conventional steering system is combined with an overriding gear, so that steering interventions can be made independently of the driver's steering demand. The properties of a steer-by-wire steering system are achieved most extensively in this way.
In these gears, play is unwanted, since it makes the steering feel worse, makes the steering interventions less precise, and moreover it makes itself felt unpleasantly in the form of “cracking noises” when there is a change in the direction of rotation.
To avoid play in the steering gears named above, it is known from German Patent Disclosure DE-OS 198 22 478 A1 to support the pinion of a worm gear axially displaceably and to prestress it resiliently in the axial direction. As a result, any play that exists in the steering gear is not reduced; instead, the occurrence of the aforementioned “cracking noises” is merely partly prevented, since after a change in the direction of rotation, when the pinion strikes the worm wheel it can deviate in the axial direction, thus lessening the impact. A disadvantage of this steering gear is that the precision of the steering motion suffers, and a compensation in length is necessary between the pinion shaft and the electric motor that drives the pinion shaft.
From International Patent Disclosure WO 99/11502, it is also known to support the pinion shaft of a worm gear of an electrical servo unit in an eccentric sleeve, so that in assembly the play of the worm gear can be adjusted. A disadvantage of this version is that wear of the gear wheels and/or of their bearing can increase play over the course of time, so that the cracking noises increase.
From Japanese Patent Disclosure JP-OS 10 281 235 A, it is known for the pinion of an electrically driven worm gear to be mounted in prestressed form in an elastic bearing formed by a rubber O-ring disposed between the pinion bearing and the housing. In this gear, no play occurs, and gear wear is automatically compensated for. However, it is disadvantageous in this version that an O-ring ages over time, so that the prestressing decreases. Moreover, the pinion can be deflected not only in the radial direction but also in a tangential direction, which lessens the steering precision.
From Austrian Patent Application Serial No. 199 44 133.2 (filed in Austria on Sep. 15, 1999) of ZF Lenksysteme GmbH, which had not yet been published by the filing date of the present application, an electrical servo unit is known in which the shaft on which the pinion of a worm gear is secured is supported at three points. One of the three bearings is displaceable in the radial direction. Exerting a spring force in the radial direction elastically deforms the shaft, thus assuring play-free engagement of the pinion with the worm wheel. However, in addition to the torsional stresses resulting from the operation of the steering gear, the shaft must also absorb a revolving bending stress.
The object of the present invention is therefore to furnish a self-adjusting gear for a vehicle steering system, which is play-free, which at least does not increase the stresses on components, and which assures high steering precision.
This object is attained according to the invention by a gear for a vehicle steering system, having a pinion disposed on a shaft in a manner fixed against relative rotation, having a gear wheel that meshes with the pinion, the pinion and the gear wheel being prestressed in the radial direction, and in which the shaft is supported pivotably in the radial direction.